A trial of oral Memantine taken alone or concurrently with an antiretroviral regimen in subjects with ADC stage 1, 2, or 3. The starting dose of Memantine will be 10mg/day; the dose will be increased by 10 mg/day every week to a maximum dose of 40 mg/day (on TID basis) or to maximum tolerated dose. Subjects must be antiretroviral free for at least 6 weeks, or have had prior stable treatment with FDA approved antiretroviral therapy (e.g. Nucleoside, protease inhibitor), alone or in combination, for at least 6 consecutive weeks prior to study entry. Subjects will be asked to maintain their concurrent antiretroviral regimen through out the primary evaluation period for 16 weeks. Following the 16 week evaluation period, there will be a 4 week washout prior to reassessment. Then all subjects successfully completing 16 weeks of therapy including those formerly receiving a placebo will be re-escalated to their MTB or the 40 mg/day dose. This study design closely follows, and the statistical portion is based on, ACTG 162, a successfully completed phase I/II study of high or low dose Nimodipine for AIDS Dementia Complex. (ACTG 301)